1. Field of the Invention
The present invention relates generally to serial radiographic systems and more particular to an apparatus for obtaining a visible radiation image by use of a stimulable phosphor sheet.
2. Discussion of Background
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays or ultraviolet rays, they store a part of the energy of the radiation. Thereafter, when the phosphor is exposed to a stimulating ray such as visible light, light is emitted from the phosphor in the pattern of the stored radiation energy. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
A serial radiographic system adapted especially for using the stimulable phosphor is described in U.S. Pat. No. 4,236,078, entitled, METHOD AND APPARATUS FOR RECORDING AND REPRODUCING A RADIATION IMAGE, issued to Kotera et al. This patent discloses a method and apparatus of radiation image recording and reproducing which employs a radiation image storage panel including a stimulated phosphor for medical diagnosis. The stimulable phosphor is first exposed to radiation to have a radiation image stored therein and is then scanned with a stimulating ray which causes it to emit light in the pattern of the stored image. The light emitted from the stimulable phosphor upon stimulation thereof is photoelectrically detected and converted to an electric signal which is processed as desired to reproduce a visible image of a quality suitable for viewing and diagnostic purposes.
In the radiation image recording and reproducing method described above, the final visible image may be reproduced in the form of a hard copy or may be displayed on a cathode ray tube. The stimulable phosphor sheet used in this method may take various forms such as a panel, drum or the like, and is hereinafter generally referred to as an imaging plate (IP). For reasons of economy, it is desirable that the IP be used repeatedly in many separate radiographic operations, such as 1,000 operations.
Two problems have been posed in reusing the stimulable phosphor sheet in the course of the radiation image recording and reproducing systems and processes as described above.
One problem encountered in reusing the stimulable phosphor sheet repeatedly has been that a part of previously stored radiation image remains stored in the stimulable phosphor sheet after use and inconveniently cause noise to occur in the visible image reproduced from the stimulable phosphor sheet when it is reused. Theoretically, the radiation energy of the radiation image stored in the stimulable phosphor sheet should disappear when the sheet is scanned with a stimulating ray of a sufficient intensity to release sequentially the radiation energy stored as light emission. However, in actuality, the stored radiation energy cannot be completely eliminated only by the scanning of the stimulating ray. Therefore, a first erasing in which the radiation image previously stored in the stimulable phosphor is to be erased at high illumination for a long length of time by use of a large-scale device can be carried out before the stimulable phosphor sheet is fed in radiography.
Another problem has been that a stimulable phosphor contains traces of radioactive isotopes such as .sup.226 Ra and .sup.40 K, which emit radiation and cause the stimulable phosphor sheet to store the emitted radiation energy even when the sheet is not being used in radiography. These traces of radioactive isotopes also constitute a cause of the noise developing in the reproduced visible radiation image. Further, a stimulable phosphor sheet is also affected by environmental radiation such as cosmic rays and X-rays emitted from other X-ray sources and stores the energy thereof. These types of radiation energy also causes noise to appear in the visible radiation image reproduced therefrom. In order to erase the stored radiation energy arising due to such extraneous sources, a second erasing can be conducted in a short length of time immediately before the next radiography operation by a small-scale erasing device positioned in the radiographic systems.
Generally it requires several seconds for the second erasing device to erase the undesirable radiation energy stored in a stimulable phosphor sheet, which is emitted from the radioactive isotopes contained in a stimulable phosphor, cosmic rays or other X-ray sources, immediately before the next radiograph being taken on the stimulable phosphor sheet.
In the prior art system, a transport system is controlled to cause the stimulable phosphor sheets to be transported intermittently and to disable selectively feed of unexposed stimulable sheets from a supply magazine to an exposure station for radiography until the exposed stimulable sheet is fed to the receiving magazine. While the radiographic operation is conducted on the stimulable phosphor sheet fed from the supply magazine by the transport system, the following unexposed sheet is held prior to passing over the erasing device or feed to the transport system. Feed of the following unexposed stimulable phosphor sheet to the transport system is actuated responsive to the signal generated when the exposed stimulable phosphor sheet is fed to the receiving magazine.
Therefore, there is the disadvantage that it is not possible to take a plurality of radiographs on the stimulable phosphor sheets successively for a short duration.
Further it is impossible to take accurate radiographs timely on the stimulable phosphor sheet due to the intermittent feed of the subsequent sheets and interval therebetween.